Water oxidation catalyst including cobalt molybdenum

ABSTRACT

A process for oxidizing water using hydrated cobalt molybdenum is disclosed. A plurality of hydrated cobalt molybdenum nanoparticles are supported on an electrode and are able to catalytically interact with water molecules generating oxygen. The catalyst can be used as part of an electrochemical or photo-electrochemical cell for the generation of electrical energy.

FIELD OF THE INVENTION

The invention relates to a process and apparatus of using cobalt molybdenum as a catalyst for the electrochemical and photochemical-electrolysis of water, and in particular to a process and apparatus using hydrated cobalt molybdenum as a catalyst for the photochemical-oxidation of water.

BACKGROUND OF THE INVENTION

Hydrogen has long been considered an ideal fuel source, as it offers a clean, non-polluting alternative to fossil fuels. One source of hydrogen is the splitting of water into hydrogen (H₂) and oxygen (O₂), as depicted in equation (1). 2H₂O→O₂+2H₂  (1)

In an electrochemical half-cell, the water-splitting reaction comprises two half-reactions: 2H₂O→O₂+4H⁺+4e ⁻  (2) 2H⁺+2e ⁻→H₂  (3) and hydrogen made from water using sunlight prospectively offers an abundant, renewable, clean energy source. However, the oxygen evolution half reaction is much more kinetically limiting than the hydrogen evolution half reaction and therefore can limit the overall production of hydrogen. As such, efforts have been made to search for efficient oxygen evolution reaction (OER) catalysts that can increase the kinetics of OER and increase the production of hydrogen from water. In particular, oxides of ruthenium and iridium have previously been identified. However, as they are among the rarest elements on earth, it is not practical to use these catalysts on a large scale. Therefore, improved OER catalysts would be very useful in the development of hydrogen as an alternative fuel source.

SUMMARY OF THE INVENTION

In one aspect there is disclosed a process for oxidizing water to produce oxygen. The process includes placing water in contact with hydrated cobalt molybdenum, the hydrated cobalt molybdenum catalyzing the oxidation of water and producing oxygen. The hydrated cobalt molybdenum can be a plurality of hydrated cobalt molybdenum nanoparticles which may or may not be attached to an electrode with an electrical potential applied between the electrode and the water to generate oxygen.

In a further aspect of the invention, a cell for oxidizing water to produce oxygen is disclosed. The cell comprises water and hydrated cobalt molybdenum, where the hydrated cobalt molybdenum catalyzes the oxidation of water and produces oxygen. The cell may further comprise a container to hold the water.

In yet a further aspect, a photo-sensitizer may be added to the water in contact with hydrated cobalt molybdenum and the water plus hydrated cobalt molybdenum plus photo-sensitizer mixture exposed to electromagnetic radiation. In this aspect, the photo-sensitizer provides an electrical potential between the hydrated cobalt molybdenum and the water. In some instances, the photo-sensitizer may be a ruthenium-tris(2,2′-bipyridal) compound, such as ruthenium-tris(2,2′-bipyridal) chloride.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of the proposed mechanism by which cobalt molybdenum (CoMoO₄) serves as a water oxidation catalyst for converting water to its elemental components with electrons transferred to a sacrificial electron acceptor such as [Ru(bpy)³]³⁺ and S₂O₈ ²⁻;

FIG. 2 is a pair of scanning electron microscopy (SEM) images of hydrated CoMoO₄ nanoparticles and anhydrous CoMoO₄ particles;

FIG. 3 is an EDX plot of the hydrated CoMoO₄ catalyst material;

FIG. 4 is an XRD plot of hydrated CoMoO₄ nanoparticles and anhydrous CoMoO₄ particles;

FIG. 5 is a graphical representation of cyclic voltammetry traces using a scan rate of 5 mV/s for CoMoO₄-carbon paste electrode for one cycle, after 10 cycles and for a CoWO₄ electrode after 10 cycles;

FIG. 6 is a graphical representation FIG. 7 is a plot of the electrochemical performance including the over potential versus the current density for hydrated CoMoO₄ and anhydrous CoMoO₄;

FIG. 7 is a graphical representation of percent oxygen produced as a function of time for a working potential applied to an ITO electrode coated with hydrated CoMoO₄ catalyst material;

FIG. 8 is a graphical plot of TGA data for hydrated CoMoO₄.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure provides a method, apparatus and/or catalyst composition for the oxidation of water to generate oxygen gases. The method includes providing a hydrated cobalt molybdenum (CoMoO₄) catalyst material and adding the catalyst to water under a condition effective to generate oxygen. In one embodiment, the method further includes exposing the water, which contains the catalyst, to light radiation to generate oxygen gases.

A “catalyst” as used herein, means a material that is involved in and increases the rate of a chemical electrolysis reaction (or other electrochemical reaction) and which itself, undergoes reaction as part of the electrolysis, but is largely unconsumed by the reaction itself, and may participate in multiple chemical transformations. A catalytic material of the invention may be consumed in slight quantities during some uses and may be, in many embodiments, regenerated to its original chemical state. The reaction may include a water oxidation or oxygen evolution reaction.

In one aspect a water oxidation catalyst or an oxygen evolution catalyst includes hydrated cobalt molybdenum that splits water into oxygen and hydrogen ions.

In a further aspect there is disclosed an electrode for electrochemical water oxidation splitting water into oxygen and hydrogen ions that includes a substrate and an active material in contact with the substrate. The active material includes hydrated cobalt molybdenum.

In one aspect, the hydrated cobalt molybdenum may be combined with conductive particles such as carbon black and may also include a binder such as NAFION®, a sulfonated tetrafluoroethylene based fluoropolymer copolymer sold by DuPont. The combined material may be attached to an electrode substrate using any method known to those in the art. Various electrode substrates may be utilized that are capable of conducting current such as for example, glassy carbon, carbon black or other materials.

The catalyst can include a plurality of hydrated cobalt molybdenum nanoparticles. In some instances, the nanoparticles are uniform in size and can have an average particle size of less than 100 nm. In one embodiment, the hydrated cobalt molybdenum is attached to an electrode using any method known to those in the art. For example for illustrative purposes only, absorption techniques, adhesives, deposition techniques and the like can be used to attach the hydrated cobalt molybdenum to the electrode.

In some instances, the electrode can have channels and water can be brought into contact with the catalyst at a rate that allows the water to be incorporated into the electrode channels. In addition, the electrode can be in an aqueous solution and/or be part of an electrochemical cell and/or part of a photo-electrochemical cell, which may or may not include a container.

The container may be any receptacle, such as a carton, can or jar, in which components of an electrochemical device may be held or carried. A container may be fabricated using any known techniques or materials, as will be known to those of ordinary skill in the art. The container may have any shape or size, providing it can contain the components of the electrochemical device. Components of the electrochemical device may be mounted in the container. That is, a component, for example, an electrode, may be associated with the container such that it is immobilized with respect to the container, and in some cases, supported by the container.

In some instances, an electrochemical cell containing an embodiment of the present invention offers a highly efficient method of splitting water using solar illumination, without the need for an applied potential. Upon oxidation of water at a photo-anode, hydrogen protons are generated which are then reduced to form hydrogen gas at a counter electrode. In addition, the oxygen and hydrogen generated from the cell can be passed directly to a fuel cell to generate further power.

In a further embodiment, the electrochemical cell can be driven either by a photo-anode such as a dye sensitized semiconductor or an external potential. The dye sensitized semiconductor acts as a chemical/photo-electrical relay system. For example and for illustrative purposes only, FIG. 1 illustrates a sequence of electron transfer that can occur in a photo-electrical relay system. Examples of such relay systems include ruthenium N-donor dyes such as ruthenium polypyridal dyes that can absorb visible light and accept electrons from a hydrated cobalt molybdenum catalyst material and thereby assist in the oxidation of water that is in contact with the catalyst. In some instances, the photo-sensitizer can be a ruthenium-tris(2,2′-bipyridyl) compound such as ruthenium-tris(2,2′-bipyridyl) chloride.

The invention is further described by the following examples, which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the invention defined in the claims.

EXAMPLES Example I

Preparation of Hydrated CoMoO₄

Starting materials of Co(NO₃)₂.6H₂O (Mw=291.03 g/mol) and Na₂MoO₄.2H₂O (Mw=241.95 g/mol) were purchased from Sigma-Aldrich and used directly without further purification. In a typical synthesis a (0.5M) Na₂MoO₄ solution was added drop-wise into a (0.5M) Co(NO₃)₂ solution with strong agitation. Following the reaction, the solution mixture was rinsed with water on a centrifuge and the particles were then washed with ethanol prior to drying overnight in an oven at 35° C. The final product was a purple powder material.

A final powder product was examined by SEM as shown in FIG. 2. It can be seen in the Figure that the hydrated CoMoO₄ particles had a smaller size in comparison to the anhydrous particles. The powder particles were also subjected to energy dispersive x-ray (EDX) analysis as depicted in FIG. 3 with an average particle size of less than 100 nm confirmed along with the presence of cobalt, molybdenum, and oxygen. The analysis indicates Cobalt:Molybdenum ratio of 1:1 and a CoMoO₄:H₂O ratio of 1:4.

XRD data as shown in FIG. 4 is shown for hydrated CoMoO₄ and anhydrous CoMoO₄. The data indicates that a CoMoO₄:H₂O ratio may be from 1:1 to 1:3 for the hydrated COMoO₄ material.

Example II

Cyclic Voltammetry (CV) of CoMoO₄

Carbon paste electrodes were prepared by grinding CoMoO₄ nanoparticles produced according to Example I above with carbon paste (BASI, CF-1010). The CoMoO₄-loaded carbon paste was then loaded onto an electrode body (BASI, MF-2010) and sanded to produce a working electrode. Alternatively the electrodes were produced by combining CoMoO₄ particles as produced above with carbon black using Nafion as a binder material. The material was then drop cast onto a glassy carbon electrode.

The CV studies were performed in a simple 3-electrode cell with Ag/AgCl and Pt wire as reference and counter electrodes, respectively. The electrolyte had a pH of 8 and was obtained with a phosphate buffer at concentrations of 50 and 200 mM. Typical scan rates were 5 and 25 mV/s.

Cyclic voltammogram traces for the CoMoO₄ particles after one cycle, 10 cycles and for CoWO₄-loaded carbon paste electrodes are shown in FIG. 5. As shown in the figure the hydrated CoMoO₄ particles after 10 cycles have an increased performance at an applied potential of 1.1 Vin comparison to particles of CoMoO₄ particles on a first cycle and particles of CoWO₄ following 10 cycles.

Tafel plot measurements, as shown in FIG. 6 of hydrated and anhydrous CoMoO₄ show that hydrated CoMoO₄ has significantly better performance per unit of surface area than anhydrous CoMoO₄ under the same conditions and applied overpotential. The performance characteristics of the hydrated CoMoO₄ at a pH of 8, a desirable range of pH for an electrochemical cell for splitting water indicates an improved electrochemical catalyst for splitting water than may be produced in a large scale using a hydrothermal reaction.

Example III

Deposition of CoMoO₄ on Indium Tin Oxide Electrode

Indium tin oxide (ITO) electrodes were selected for additional water oxidation testing with ITO glass slides measuring 25×75 mm purchased from SPI Supplies (#6415-CF). ITO electrodes were produced by cutting an ITO glass slide into four equal pieces using a diamond blade, each ITO glass slide producing four ITO electrodes.

To immobilize or attach hydrated CoMoO₄ nanoparticles on an ITO electrode, cobalt molybdenum nanoparticles produced according to Example I were first dispersed in ethanol. A typical dispersion solution contained 10 mg of CoMoO₄ nanoparticles in 1 ml of ethanol. The dispersion solution was sonicated for approximately 20 minutes and the CoWO₄ nanoparticles remained well-dispersed for up to several days in the ethanol.

Deposition of CoMoO₄ nanoparticles onto ITO glass slides was performed using a dipping technique and a Nima Dip Coater in order to achieve uniform coating. The CoMoO₄-ITO electrodes were then baked in an oven at 150° C. for one hour.

Example IV

Verification of Oxygen Production

An air-tight H-cell was designed to quantify oxygen production in the cell. A copper rod with an alligator clip was attached at one end to hold the CoMoO₄-ITO electrode, while Ag/AgCl and platinum coils were used as reference and counter electrodes, respectively. The two chambers in this H-cell were typically filled with 35 ml of pH 8 phosphate buffer (200 mM). The electrode area was controlled by covering the undesired area with Teflon tape. The typical electrode area used in these studies was 1 cm² and scan rates of 5 and 25 mV/s were used for a potentiostatic study in which real-time monitoring of the concentration of dissolved oxygen in the electrolyte was performed. The study included a CoMoO₄-ITO working electrode being set at voltages between 0.8-1.3 V versus the Ag/AgCl reference electrode for 15 minutes under each applied potential, and the concentration of oxygen near the electrode was recorded continuously throughout the study. As shown in FIG. 7, an increase in oxygen concentration was observed at voltages as low as 1.0 V (˜200 mV overpotential) with subsequent increases in the oxygen concentration at higher over potentials of 300 and 400 mV. A drop in oxygen concentration was also observed when there was no potential applied. Without being bound by theory, this result suggests that Co²⁺ ions might have been “activated” to Co³⁺ or Co⁴⁺ for catalytic oxidation of water.

Example V

Referring to FIG. 8, there is shown a plot of the TGA analysis of the hydrated CoMoO₄ particles. The Total weight loss due to water by TGA was found to be approximately 10% with an atomic ratio of CoMoO₄:H₂O to be approximately 1:1.

In one aspect, the atomic ratio of CoMoO₄:H₂O may be from 1:1 to 1:4 as supported by the examples.

The invention is not restricted to the illustrative examples described above. Examples described are not intended to limit the scope of the invention. Changes therein, other combinations of elements, and other uses will occur to those skilled in the art. The scope of the invention is defined by the scope of the claims. 

Having described our invention, we claim:
 1. A process for oxidizing water, the process comprising: providing hydrated cobalt molybdenum; providing water; and placing the water into contact with the hydrated cobalt molybdenum, the hydrated cobalt molybdenum catalyzing the oxidation of water.
 2. The process of claim 1, wherein the hydrated cobalt molybdenum is a plurality of hydrated cobalt molybdenum nanoparticles.
 3. The process of claim 2, further including applying an electrical potential between the hydrated cobalt molybdenum and the water.
 4. The process of claim 2, further including adding a photo-sensitizer to the water and exposing the water with photo-sensitizer to electromagnetic radiation, the photo-sensitizer providing an electrical potential between the hydrated cobalt molybdenum and the water.
 5. The process of claim 4, wherein the photo-sensitizer is a ruthenium-tris(2,2′-bipyridyl) compound.
 6. The process of claim 1, wherein the hydrated cobalt molybdenum has a CoMoO4: H₂O ratio of from 1:1 to 1:4. 